Abstract
Abstract. In our previous works, it was demonstrated that the combined use of quantitative energy-dispersive electron probe X-ray microanalysis (ED-EPMA), which is also known as low-Z particle EPMA, and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) imaging has great potential for a detailed characterization of individual aerosol particles. In this study, extensively chemically modified (aged) individual Asian dust particles collected during an Asian dust storm event on 11 November 2002 in Korea were characterized by the combined use of low-Z particle EPMA and ATR-FTIR imaging. Overall, 109 individual particles were classified into four particle types based on their morphology, elemental concentrations, and molecular species and/or functional groups of individual particles available from the two analytical techniques: Ca-containing (38%), NaNO3-containing (30%), silicate (22%), and miscellaneous particles (10%). Among the 41 Ca-containing particles, 10, 8, and 14 particles contained nitrate, sulfate, and both, respectively, whereas only two particles contained unreacted CaCO3. Airborne amorphous calcium carbonate (ACC) particles were observed in this Asian dust sample for the first time, where their IR peaks for the insufficient symmetric environment of CO32− ions of ACC were clearly differentiated from those of crystalline CaCO3. This paper also reports the first inland field observation of CaCl2 particles probably converted from CaCO3 through the reaction with HCl(g). HCl(g) was likely released from the reaction of sea salt with NOx/HNO3, as all 33 particles of marine origin contained NaNO3 (no genuine sea salt particle was encountered). Some silicate particles with minor amounts of calcium were observed to be mixed with nitrate, sulfate, and water. Among 24 silicate particles, 10 particles are mixed with water, the presence of which could facilitate atmospheric heterogeneous reactions of silicate particles including swelling minerals, such as montmorillonite and vermiculite, and nonswelling ones, such as feldspar and quartz. This paper provides detailed information on the physicochemical characteristics of these aged individual Asia dust particles through the combined use of the two single-particle analytical techniques, and using this analytical methodology it is clearly shown that internal mixing states of the aged particles are highly complicated.
Highlights
M individual particles available from the two analytical techniques: Ca-containing (38 %), NaNO3-containing (30 %), Nearly every spring, “Asian dust” originating mostly in westsilicate (22 %), and miscellaneous particles (10 %)
After the relocation of the image field, the sample was placed in contact with the internal reflection element (IRE) crystal for the ATR-FTIR imaging measurement
The presence of IR peaks of H2O at 1620–1630 and 3300–3440 cm−1 in their ATR-FTIR spectra clearly shows that the dark particles contain water. (The assignment of IR peaks mentioned in this work are listed in Table S2 of the Supplement to aid the reader in understanding the discussion of the speciation by ATR-FTIR below.)
Summary
On 11 November 2002 when an Asian dust storm event occurred, aerosol particle samples were collected on the roof of a campus building of Hallym University located in ChunCheon, Korea. ChunCheon (37◦89 N, 127◦73 E) is a relatively small city (population: 0.26 million; area: 1116 km2) with a mostly rural character outside of the central district that is free of industrial complexes. This event was a recordbreaking episode and was reported to originate from the Gobi and Inner Mongolian deserts (Chung et al, 2003). Particles were sampled on Ag foil using a seven-stage May cascade impactor. The May impactor has, at a 20 L min−1 sampling flow, an aerodynamic cut-off of 16, 8, 4, 2, 1, 0.5, and 0.25 μm for stages #1-7, respectively. 109 particles for the stage 3 sample were analyzed in this study
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